1. Field of the Invention
This invention relates to a multi-position directional control valve, and more particularly to a spool-type valve that is operable by either a lever or a pilot valve to move a valve spool to a detented center position between two detented end positions.
2. Description of the Prior Art
Pilot operated directional control valves for directing fluid flow in a preselected direction from a valve chamber by slidable movement of a valve member, such as a spool-type valve member, in the valve chamber are known in the art. The known spool-type valve member is operable to place in a preselected configuration inlet, tank and outlet ports in fluid communication with respect to one another by sliding the valve spool to a pre-selected position in the valve chamber. A conventional pilot operated valve spool is shifted to either one of two extreme end positions by conducting pressurized fluid to one end of the valve spool and venting the opposite end of the valve spool. This shifts the valve spool to a first end position where selected inlets and outlets are interconnected in one configuration for selected directional flow through the valve chamber. To change the directional flow through the valve chamber a second configuration of inlets and outlets is arranged by shifting the valve spool to a second end position by reversing the flow of pressurized fluid to the ends of the valve spool.
The shifting movement of the valve spool, either left or right, is accomplished by operation of an electrohydraulic pilot valve as known in the art. The pilot valve is actuated to supply fluid under pressure to a selected end of the valve spool and to vent the opposite end to tank. In this manner the valve spool is shifted to either one of two extreme end positions.
With the above described pilot operated directional valve, the valve spool is shifted to a center position between the two extreme end positions by springs positioned between the ends of the valve spool and the end walls of the valve chamber. In the absence of fluid pressure at the ends of the valve spool, the centering springs bias the valve spool in a center position within the valve chamber. In this position the fluid is directed in a third pattern or configuration through the valve chamber where, for example, the inlet port is connected to a tank port and two outlet ports are blocked. Thus the valve spool is normally maintained in the center position by the centering forces exerted upon the valve spool by the end springs. Actuation of the pilot valve supplies fluid to a selected end of the valve spool to overcome the centering forces exerted upon the valve spool in order to move the valve spool from the center position to one of the extreme end positions. Constant pilot pressure must therefore be exerted upon the valve spool to maintain the end position. To return the valve spool to the center position the pilot valve is deactuated to remove the pilot pressure from either end of the valve spool. The centering springs are then free to act on the valve spool to return the spool to the center position.
It is also known with pilot operated valves to overcome the spring forces exerted upon the valve spool by displacing the valve spool through a manually operated lever. The lever is pivotally connected to the valve spool and by shifting the lever either left or right, the valve spool is shifted in a corresponding direction. However, with such an arrangement, the operator must maintain the lever in the shifted position to overcome the spring forces normally urging the valve spool to the center position. This has the disadvantage of leaving the operator with only one free hand to manipulate other devices, such as another control valve for example in the operation of a hydraulically controlled vehicle. For vehicles and other devices with more than two controls, such an arrangement provides for an inefficient operation.
Three-position, lever operated and detented directional control valves are known and are operable to shift a valve spool by operation of a lever to one of three positions, either of two end positions or a center position. The lever is connected to the valve spool in a manner to permit the operator to shift the valve spool to one of three positions. A detent mechanism within the valve housing maintains the valve spool in the selected position so that the operator may release his hand from the lever to operate other hand controlled devices.
Generally the detent mechanism includes a pair of ball bearings which are carried by the valve spool and are biased by a compression spring inserted between the ball bearings. The ball bearings are mounted transverse to the spool. The spring normally tends to urge the ball bearings into one of three annular detents or grooves formed in a sleeve surrounding the spool and fixed in the valve housing. Each detent corresponds to one of the three valve positions. When the ball bearings are positioned oppositely of a selected detent by movement of the valve spool, the spring moves the ball bearings into engagement with the detent to lock the valve spool in a preselected position. The valve spool is shifted to engage the ball bearings with a selected detent by operation of the lever. Movement of the lever overcomes the detent spring force and shifts the valve spool to another position.
In a directional control valve there is need for apparatus to combine the features of pilot-operated control valves and internally detented control valves. While it has been suggested by the prior art devices to pilot operate a directional control valve to move a valve spool between two end positions of travel, the prior art devices require a constant force, either hydraulic or manual, to be exerted upon the valve spool to hold the spool at either end position. Even though known internally detented control valves are operable to provide three positions for the spool without the need for a constant force to be exerted upon an end of the spool to hold it in position, this arrangement is not pilot operable in all three spool positions. Therefore, there is need for a three position, internally detented control valve that is pilot operated by a conventional electrohydraulic pilot valve. Such a device would provide hands off operation of the control valve and maintain the valve spool in either end position or the center position without requiring a preselected pressure differential acting on the valve spool or manually maintaining the spool in a preselected position.